1. Field of the Invention
The present invention relates to microelectromechanical systems (MEMS) and, in particular, relates to the fabrication of electrically isolated MEMS devices using plating techniques.
2. Discussion of the Related Art
Microelectromechanical systems (MEMS) components are being progressively introduced into many electronic circuit applications and a variety of micro-sensor applications. Examples of MEMS components are electromechanical motors, radio frequency (RF) switches, high Q capacitors, pressure transducers and accelerometers. In one application, the MEMS device is an accelerometer having a movable component that, in response to acceleration, is actuated so as to vary the size of a capacitive air gap. Accordingly, the current output of the MEMS device provides an indication of the strength of the external stimulus.
One current method of fabricating such components, often referred to as surface micro-machining, uses a sacrificial layer, such as silicon dioxide, that is deposited and bonded onto a substrate, such as single crystal silicon which has been covered with a layer of silicon nitride. A MEMS component material, for example polycrystalline silicon, is then deposited onto the sacrificial layer, followed by a suitable conductor, such as aluminum, to form an electrical contact with the ambient environment. The silicon layer is then patterned by standard photolithographic techniques and then etched by a suitable reactive ion etching plasma or by wet chemistry to define the MEMS structure and to expose the sacrificial layer, which may comprise silicon dioxide. The sacrificial layer is then etched to release the MEMS component. This leaves only a single material, the structural material.
One significant disadvantage associated with current surface fabrication techniques involves the lack of electrical isolation that is achieved. The present inventors have discovered that a MEMS device may be used as a current or voltage sensor, in which the device may receive high voltages at one end of the device, and output an electrical signal at the other end of the device to, for example, a sensor. The output could be a function of the capacitance of the MEMS device, as determined by the position of a movable MEMS element with respect to a stationary element. However, because the entire movable MEMS element achieved using conventional surface fabrication techniques is conductive, the input and output ends of the MEMS device are not sufficiently isolated from one another, thereby jeopardizing those elements disposed downstream of the MEMS output.
Another significant disadvantage associated with current surface fabrication techniques is that the process is inherently limited to thin structural layers (on the order of 1 to 2 xcexcm) due to stresses which may be introduced during the fabrication. The thinness of the layers limits the amount of capacitance that can be obtained in the sensor portion of the MEMS device, and thus limits the magnitude of any output signal. This in turn limits the overall resolution obtainable
It is therefore desirable to provide a method for fabricating a MEMS device using surface fabrication techniques having greater thickness than that currently achieved to enhance sensitivity, while providing sufficient electrical isolation for the device.
The present inventors have recognized that a MEMS device may be fabricated using an insulating material, a sacrificial material, a mold material, and a conducting mechanical structural layer that may be plated onto an insulating substrate.
In accordance with one aspect of the invention, a method for fabricating a MEMS device, comprising the steps of providing a substrate having an upper surface, and depositing a sacrificial layer onto the upper surface of the substrate. A nonconductive layer is then deposited onto the upper surface of the sacrificial layer. Next, a mold is deposited onto the substrate, wherein the mold has at least one void aligned with the insulating layer. A-conductive material is then deposited into the at least one void to form conductive elements extending from the nonconductive layer. Finally, the mold and sacrificial layer are removed to release a movable element including the nonconductive layer and conductive layer from the substrate.
The conductive material may be electroplated or electrolessplated onto the nonconductive layer.
All of the aforementioned aspects are not necessary to carry out the invention. Furthermore, these and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.